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Cold Water Fermentation: Pasteurizing Straw Without Heat

Straw is a very common commercially popular substrate for the cultivation of a wide variety of different edible and medicinal mushroom species, and thanks to the folks over at Radical Mycology, I am now aware of an alternative method of preparing the straw for colonization with mycelium. There are many fungi that grow well in pasteurized straw, including but certainly not limited to many species comprising the oyster mushroom genus (Pleurotus spp.) as well as paddy straw mushrooms (Volvariella volvacea) and shaggy manes (Coprinus comatus) among others. For the most part, straw is easy to obtain and process and this has made it one of the ideal material choices for commercial mushroom growers. To the opportunistic and innovative mushroom grower, straw makes a prime candidate for experimenting as a suitable substrate for mushroom cultivation. In order to successfully grow mushrooms using straw, it must be pasteurized or ‘cleaned’ of most if not all harmful fungal spores, bacteria or other pathogens before it can be mixed with mushroom spawn.

Now before I can continue there is a discrepancy which must be firmly addressed. Pasteurization and sterilization, despite sounding like quite similar processes, are in reality fundamentally different and result in different outcomes. By sterilizing a substrate (if the process was successful) you have managed to kill every living thing on the surface of that material that you have sterilized. This can be compared to starting off with a ‘blank canvas’ where anything that you introduce onto the substrate ought to be the only living thing in contact with that material. Pasteurization, on the other hand, is slightly different. Renowned mycologist Paul Stamets says it best in his book Growing Gourmet and Medicinal Mushrooms: “Pasteurization selectively kills off populations of temperature sensitive microorganisms. The population intact represents little competition to the mushroom mycelium for approximately 2 weeks, giving ample opportunity for the mushroom mycelium to colonize.”

Essentially, pasteurization removes many of the harmful bacteria and fungal spores that may compete with the fungi that you are attempting to introduce to the substrate. Additionally, the heat-tolerant microorganisms remaining in the straw after pasteurization help to prevent contamination from unwanted microorganisms (a.k.a. ‘baddies’) because their niches are already being occupied by the harmless or inert populations of microorganisms. These remaining ‘goodies’ may even aid in/assist with the growth and development of the fungus itself. There are three different methods that I will discuss here that can be done to pasteurize straw: hot water immersion, steam pasteurization and pasteurization through natural fermentation.

Hot water immersion involved steeping the straw (often pre-cut to approximately 1-4 inch lengths to make handling easier) in 160-180 °F water for at least 1 hour but not more than 2. It helps to have something weighing down on the straw to help keep it submerged. The temperature of the water, as discussed above, kills heat-sensitive fungi and bacteria that may be harmful to your mycelium but leaves beneficial or inert microorganisms unscathed. After the hot water bath, the straw is transfered to perforated racks where it can cool and drain before it can be further processed. Steam pasteurization results in exactly the same outcome except that high-pressure steam is released into heavily insulated rooms or containers containing pre-moistened straw arranged in layers on drainage racks. This option, in my humble opinion, is the least practical for the home mushroom grower as specialized equipment is pretty much entirely necessary. Considering that hot water immersion can be performed in a pot, on any stove or heat source, in any home (or fire pit) I believe it to be the more economical and less labor-intensive method. Edit: another important piece of information: after being pasteurized, the straw can be safely handled with your hands (as long as they are reasonably clean) to be drained and stuffed into containers without overly increasing the risk of contamination. If you want to be extra careful you could wear rubber/latex gloves although either way you should expect not to have perfect but still fairly high successful colonizations.

The last technique, pasteurization through natural fermentation, is by far the least energy intensive but takes considerably longer than the other methods. (Check out the original article right here by Radical Mycology for more information) The principal here is that by immersing the straw in cool, fresh water (chlorinated tap water is not recommended to be used when working with fungi due to it’s anti-microbial and anti-fungal properties; rain, well or filtered water is preferred) for 3 or 4 days which allows anaerobic microorganisms to flourish and multiply on the substrate, creating a rich biological complexity which kills fungal spores or aerobic bacteria. Don’t worry too much about the ‘appetizing’ (hint extreme sarcasm) aroma that emanates from the container after this fermentation period as straining the soaking straw and exposing it to oxygen not only gets rid of the odor but also then kills all of the oxygen-hating anaerobic microorganisms, leaving you with a ‘blank canvas’ which you can then inoculate with your mushroom mycelium.

This last method seems the most sustainable and energy efficient option for pasteurizing particularly large volumes of straw or other bulk substrates for the purpose of mushroom growing, but as Stamets once again says later on the same chapter “I hesitate to recommend it [pasteurization through natural fermentation] over the other procedures described here.” Staments makes this comment on the basis that this method may not be as successful to ‘clean’ the straw and subsequently result in more contamination than the two other heat-treatment methods. Despite this, depending on the condition of your substrate, your working environment and your personal view of cleanliness when working with mushrooms, your results may vary. I would still consider this just as much of a legitimate method as all the others purely on the basis that it can be conducted by virtually everyone living anywhere and therefore makes the possibility of growing mushrooms for food or medicine more plausible for the average individual; a principal that I am pretty much all about promoting.

For additional information pertaining to pasteurizing straw and other bulk substrates for mushroom culture, check out this and this. Earlier this week I tried out the hot water immersion method (being careful to watch the thermometer to make sure the water temperature didn’t leave the 160-180 °F range) and will report on the results as soon as they happen. I mixed the pasteurized straw with an old jar of Pleurotus columbinus mycelium which I broke into pieces with my clean hands (I washed them several times with soap every few minutes during this whole procedure) and mixed them with the drained and cooled pasteurized straw before packing the mixture into jars and topping them with a double layer of aluminum foil. They will now incubate at room temperature (roughly 70 °F) and I’ll await some mushroom magic. Stay tuned for fun!